Secondary batteries can be repeatedly used by means of charging and are thus used as power sources for various electric apparatuses. Moreover, in recent years, high-capacity secondary batteries have also been developed and used as power sources for hybrid and electric vehicles. Furthermore, various improvements have been made on a charging apparatus for charging a secondary battery and on a charging apparatus with a discharging function.
For example, one capability that is required for charging apparatuses is a reduction in charging time. The reduction in charging time is particularly important for a charging apparatus that charges a secondary battery in an electric vehicle. As the use of electric vehicles becomes widespread, the expectation is that secondary batteries will be supplied with energy in a way similar to that in which gasoline powered vehicles are supplied with gasoline at service stations. In this case, the user of the vehicle moves the vehicle to a charging station, where the user charges the secondary battery in the vehicle.
Japanese Patent Laid-Open No. 5-207668 discloses a charging apparatus intended to complete charging a secondary battery in a time equivalent to that required to supply gasoline to a gasoline-powered vehicle at a service station. The charging apparatus includes a rectifier that converts commercial alternating current power into direct current power, a secondary battery for buffering, and a charger that controls low-current charging for and discharging from the secondary batter for buffering. If the secondary battery in the vehicle is not charged, direct current converted by the rectifier is input to the secondary battery used for buffering via the charger to charge the secondary battery used for buffering with low current. If the secondary battery in the vehicle is charged, the direct current power charged in the secondary battery used for buffering is input to the secondary battery in the vehicle via the charger. Thus, the secondary battery in the vehicle is charged with high current, that is, the secondary battery is rapidly charged.
On the other hand, a charging/discharging apparatus is used in, for example, a stage of developing a secondary battery or a process of manufacturing a secondary battery, to test the charging and discharging characteristics of the secondary battery. For example, Japanese Patent Laid-Open No. 9-159738 discloses a charging/discharging apparatus including a charging power source, an electronic loading device, and a secondary battery for buffering. In the charging/discharging apparatus, to charge a secondary battery used for testing, the charging power source and the secondary battery used for buffering are connected in series with the secondary battery used for testing. To discharge the secondary battery used for testing, the electronic loading device and the secondary battery used for buffering are connected to the secondary battery used for testing.
According to the charging/discharging apparatus disclosed in Japanese Patent Laid-Open No. 9-159738, electric energy generated by discharge from the secondary battery used for testing is accumulated in the secondary battery used for buffering. The electric energy accumulated in the secondary battery used for buffering can be used to charge the secondary battery used for testing. As a result, the electric energy can be effectively utilized.
However, the above-described conventional charging apparatus and charging/discharging apparatus pose problems described below.
The charging device for the secondary battery in the electric vehicle disclosed in Japanese Patent Laid-Open No. 5-207668 is connected to the vehicle and thus expected to be used outdoors. Thus, the charging apparatus is likely to be affected by ambient temperature.
The output characteristics and lifetime of the secondary battery generally vary depending on environmental temperature. This also applies to the secondary battery used for buffering provided in the charging apparatus. That is, in a low temperature environment as in winter, the direct current resistance of the secondary buffer for buffering rises to reduce the maximum current that can be output. The reduced maximum current for the secondary battery used for buffering increases the time required to charge the secondary battery. In contrast, in a high temperature environment, as in summer, the secondary battery used for buffering is left in such a high temperature atmosphere. When left in a high temperature atmosphere, the lifetime of the secondary battery used for buffering becomes shorter.
On the other hand, for the charging/discharging apparatus, which tests the charging/discharging characteristics of the secondary battery, the electrical characteristics of electric and electronic components used in the charging/discharging apparatus need to be stable in order to allow the charging/discharging characteristics of the secondary battery to be accurately evaluated.
However, the charging/discharging apparatus disclosed in Japanese Patent Laid-Open No. 9-159738 has disadvantages described below in connection with on accurate evaluation of the charging/discharging characteristics. During charging/discharging tests of the secondary battery used for testing, the secondary battery used for buffering is also charged and discharged. When a large current flows through the secondary battery, the secondary battery generates heat due to resistance heat resulting from the internal resistance thereof. Thus, the temperature of the secondary battery used for buffering rises and the electrical characteristics of the secondary battery used for buffering changes. Consecutive tests accumulatively raise the temperature of the secondary battery used for buffering. As a result, testing conditions vary for each test, preventing the secondary battery used for testing from being accurately evaluated. To avoid the adverse effect of the rise in the temperature of the secondary battery used for buffering, it is possible to wait for the temperature of the secondary battery used for buffering to become lower before the next test is started. However, in this case, a long time is required to wait for the temperature to become lower, thus reducing testing efficiency. In particular, if the charging/discharging apparatus is used in the process of manufacturing the secondary battery, the manufacturing efficiency of the secondary battery is reduced.
That is, for a charging/discharging apparatus having a secondary battery used for buffering, managing the temperature of the secondary battery used for buffering is important, as is the case with the charging apparatus.
As an outer cover for the secondary battery, a can is often used which is obtained by shaping a metal plate having a thickness about 0.3 to 0.5 mm into a shape like that of a cylinder or a box. However, if the can is used as an outer cover, heat outside the can has difficulty being transmitted to an internal battery element. Furthermore, the thermal capacity of the can cannot be neglected. Moreover, producing a secondary battery having a thickness of at most 3 mm is difficult owing to processing limits on metal plates. For the above-described reasons, it is difficult to control the temperature of the battery element inside the can over a short span or to control the temperature of the entire battery element to a uniform value.
Thus, an object of the present invention is to provide a charging apparatus and a charging/discharging apparatus which allows the temperature of the secondary battery used for buffering to be easily managed.